Multiple lead screw compressor

ABSTRACT

A rotor structure is disclosed for a fluid compressor of the character having a pair of complementary, intermeshing rotatable rotors for transferring and compressing a fluid from an inlet into the compressor to an outlet from the compressor. The rotor includes helical land and intervening helical groove means for intermeshing with complementary helical land and groove means of a complementary rotor. The helical land and groove means includes a first portion having a first constant helix angle and a second portion having a second constant helix angle which is different from the first helix angle. The two helical portions are disposed on shaft means and meet in a transition area intermediate the opposite ends of the shaft means and in which area the lands and grooves are disposed in mating relationship. One of the helical portions is integral with the shaft means, and the other helical portion is defined by a separate component adapted to be mounted on the shaft means and suitably interconnected with the one helical portion.

United States Patent 1191 Caffrey MULTIPLE LEAD SCREW COMPRESSOR [75]Inventor: Terence Caffrey, Kent, Ohio [73] Assignee: Davey CompressorCompany, Kent,

Ohio 1 [22] Filed: May 14, 1973 [21] App]. No.: 359,665

4 Related U.S. Application Data [63] Continuation of Ser, No. 168,023,Aug. 2, 1971,

abandoned.

52 u.s.c 1. ..4l8/9,4l8/20l 51 Int. Cl. F01C1/16,F04C 1/10,F04c 17/12 58FieldofSearch 418/9, 197, 201,202, 203

[56] References Cited UNITED STATES PATENTS 3 ,424',373 1/1969 Gardner418/201 2,652,192 9/1953 Chilton 418/197 2,369,539 2/1945 0613mm 418/2031,597,411 8/1926 Kinney 418/202 2,586,842 2/1952 McCallum 418/2012,691,482 10/1954 Ungar 418/9 2,975,963 3/1961 Nilsson 418/9 2,804,2608/1957 Nilsson et al. 418/9 FOREIGN PATENTS OR APPLICATIONS 210,088

l/l924 Great Britain 418/202 [11] 3,807,911 1451 Apr. 30, 1974 PrimaryExaminerCarlton R. Croyle Assistant Examiner-John J. Vrablik Attorney,Agent, or Firm-Meyer, Tilberry & Bod

[ ABSTRACT A rotor structure is disclosed for a fluid compressor of thecharacter having a pair of complementary, intermeshing rotatable rotorsfor transferring and compressing a fluid from an inlet into thecompressor to an outlet from the compressor. The rotor includes helicalland and intervening helical groove means for intermeshing withcomplementary helical land and groove means of a complementary rotor.The helical land and groove means includes a first portion having afirst constant helix angle and a second portion having a second constanthelix angle which is different from the first helix angle. The twohelical portions are disposed on shaft means and meet in a transitionarea intermediate the opposite ends of the shaft means and in which areathe lands and grooves are disposed in mating relationship. One of thehelical portions is integral with the shaft means, and the other helicalportion is defined by a separate component adapted to be mounted on theshaft means and suitably interconnected with the one helical portion.

5 Claims, 6 Drawing Figures SHEEI 1 BF 2 [\l INVENTOR. LK TERENCECAFFREY W% 7% ya ATTOR NEYS.

PATENIED H I 3.807311 SHEET 2 [1F 2 INVENTOR.

T E RE NCE CAF FREY 77% Hay ATTORNEYS.

MULTIPLE LEAD SCREW COMPRESSOR This is a continuation, of applicationSer. No. 168,023 filed Aug. 2, 1971.

This invention relates to the art of helical screw-type compressors and,more particularly, to helical screw or rotor components for suchcompressors.

Rotary screw-type compressors have been provided heretofore andgenerally include a pair of intermeshing, complementary rotorspositioned within a compressor housing, and means for rotating theintermeshed rotors relative to one another. The compressor housingincludes an inlet opening into which fluid such as air, flows and anoutlet opening through which compressed fluid flows as a result ofrotation of the intermeshing rotors which operate to transfer andcompress the fluid from the inletto the outlet of the compressor.Compressors of this type are employed for compressing gases which,during transfer and compression, move axially along the rotors. Suchrotary screw compressors may beoperated dry or wet. In wet operatedcompressors, liquid is introduced into the compression chamber to effecta seal between the rotors. Dry compressors, on the other hand, rely uponextremely close tolerances between intermeshing lands and grooves of therotors to effect a seal therebetween. The introduction of liquids intothe compression chamber of a screw compressor, of course, provides foradditional sealing between the rotor components and serves further todissipate the heat of compression. The compression ratio which can berealistically achieved under dry operation is about 3:l or 4:1 whereas awet operated compressor may have a compression ratio of up to 8:1 or9:1.

Prior art screw compressors of the above character have generally beenconstructed with constant lead helical lands and intervening grooves onthe complementary intermeshing rotors. Such construction limits theefficiency and performance of the compressors, especially at highercompression ratios. In this respect, the discharge velocity from a screwcompressor having compressor. Efforts have been made in the past toprovide rotary screw-type compressors with rotors having lands andgrooves which continuously vary'in lead from one end of the rotor to theother. While such rotor components may provide for increasing thedischarge areaof a compressor to relieve backflow and the resultantoverworking of the compressor, it remains that a continuously variablelead rotor-is both extremely difficult and expensive to manufacture.

Backflow advantageously is relieved in accordance with the presentinvention by employing rotors or screws having helical lands andintervening grooves which have a different helix angle or lead at theintake end of the rotor from the lead thereof at the discharge end. Bythis arrangement, it is possible to provide a discharge port for a screwcompressor which is approximately 2.25 times the size of the dischargeport of a compressor of comparable size employing rotorshaving aconstant lead. Moreover, such a rotor is structurally simple andreasonably inexpensive to produce relative to a continuously varyinglead rotor..

Accordingly, it is an outstanding object of the present invention toprovide a rotor component for screw-type.

fluid compressors which provides for increasing the efficiency of aconstant lead screw compressor of a given size by permitting enlargementof the discharge port thereof to achieve a reduction in dischargevelocity without backflow and resultant additional work byv thecompressor.

A further object of the present invention is the provision of a rotorcomponent of the above character which includes helical lands andintervening grooves in portions having different leads and in each ofwhich portions the lead is constant along the length thereof.

Yet another object of the present invention is the provision of a rotorcomponent of the above character which is reasonably simple andinexpensive to manufacture. I

Still another object of the present invention is the provision of ascrew compressor rotor of the above character wherein the two helicalportions are defined by separate components, each having helical landsand intervening grooves and wherein the lead of each portion is constantalong'its length, thus facilitating the manufacture thereof.

Still another object of the present invention is the provision of arotor of the above character wherein the separate portions are adaptedto be readily interconnected to define a rotor component having helicallands and intervening grooves in portions having different leads, andwhich portions are interconnected in a manner whereby they meet in atransition area in which the lands and grooves of the two portions aredisposed in mating relationship.

The foregoing objects will in part beobviousand in part more fullypointed out hereinafter in conjunction with the following description ofthe drawing of a preferred embodiment and in which:

FIG. 1 is a plan view, partially in section, illustrating a rotaryscrew-type compressor having a pair of rotor components made inaccordance with the present invention; I

FIG. 2 is a sectional elevation of the compressor of FIG. 1, the sectionbeing taken along line 2--2 in FIG.

1, I FIG. 3 is a plan view of a male rotor made in accordance with thepresent invention;

FIG. 4 is a plan view of a female rotor made in accordance with thepresent invention;

FIG. 5 is a view in cross-section of the rotor illustrated in FIG. 3,the section being taken along line 55 in FIG. 3; and

' FIG. 6 is a schematic projection of complementary male and femalerotors made in accordance with the present invention.

Referring now to the wherein the showings are for the purpose ofillustrating preferred embodiments of the invention only and not forlimiting the same, a rotary screw-type compressor assembly 10 isillustrated in FIGS. I and 2 which is comprised of a housing 12 havingan inlet port 14 at one end thereof and an outlet port 16 at the otherend "thereof. Housing 12 includes cylindrical chambers 18 drawings ingreater detail,

in a manner to support the rotors for rotation within the housing inintermeshing relationship. ()ne of the shaft portions such as shaftportion 28, for example, projects outwardly of housing 12 for suitableinterconnection thereof with suitable drive means which is notillustrated. The intermeshing relationship between rotors 22 and 24provides for rotor 24 to be rotated in response to rotation of rotor 22through shaft portion 28. It will be understood, however, that shaftportion 32 could be extended exteriorly of housing 12 and that shaftportions 28 and 32 could be suitably interconnected such as by gearmeans to achieve rotation of rotor 24.

' Male rotor 22, as best illustrated in FIGS. 2 and 3, includes helicalland means 34 and intervening helical groove means 36. The helical landmeans of the male rotor are in the form of lobes having convex outerside surfaces 40. In accordance with the present invention, the helicalland means and intervening groove means of the male rotor is defined bytwo portions designated A Portion A extends from the inlet end of thecompressor toward transition area E, and portion B extends fromtransition area E toward the outlet end of the compressor. The helicalland and groove means of portions A and B each have a constant leadthroughout the length thereof and, as is best illustrated in FIG. 3, thelead of portion A is greater than the lead of portion B. Preferably, thelead ratio of portion A' to portion B is approximately 3:1. It will beappreciated, however, that the lead ratio can be of any value greater orlesser than 3:1 and is limited only by manufacturing or strengthconsiderations. The inner ends of portions A and B are interrelated intransition area E in a manner whereby the helical land and groove meansof portions A and B are disposed in mating relationship. Thus,continuous but angularly related helical land and groove means areprovided'which extend along the length of the rotor component.

In conjuction with the lead ratio of the helical land and groove meansof portions A and B, it is desirable to maintain the total wrap anglethereof at less than 360. More particularly, it is preferred that thetotal wrap angle be approximately 270. The total wrap anconcave outerside surfaces 46. Similarly, helical groove means 44 are complementaryto helical land means 34 of male rotor 22. Thus, rotors 22 and 24 areadapted to be disposed in side by side intermeshing reand B which meetin transition area designated by line gle, of course, is the anglecircumscribed by the helical land and intervening groove means of bothportions between the longitudinally oppositeends thereof. Similarly,portion A has a wrap angle which is defined by the angle circumscribedby the helical land and groove means thereof in extending from the endsthereof adjacent shaft portion 28 to transition area E, and portion Bhas a wrap angle which is defined by the angle circumscribed by thehelical land and groove means thereof in extending from transition areaE to the ends thereof adjacent shaft portion 26. In the preferredembodiment herein illustrated, portions A and B have a lead ratio of3:1, as mentioned above, and the wrap angle of the helical land andgroove means of each portion is approximately 135.

The structure of female rotor 24 is, of course, complementary to that ofmale rotor 22. In this respect, with reference to FIGS. 2 and 4 of thedrawing, the female rotor includes helical land means 42-and interveninghelical groove means 44. Helical land means 42 are complementary tohelicalgroove means 36 of the male rotor and, accordingly, helical landmeans 42 include lationship.

The helical land and groove means of female rotor 24 include portions Cand D corresponding respectively to portions A and B of male rotor 22.Portions C and D of the helical land and groove means are interrelatedin a transition area therebetween denoted by line F. The transitionareas of rotors 22 and 24, of course, coincide in location relative tothe opposite ends of the rotors. Since the helical land and groove meansof the male and female rotors are complementary, it will be appreciated,that the helical land and groove means of portion C have a leadcorresponding to that of portion A of male rotor 22, and that thehelical land and groove means of portion D have a lead corresponding tothe lead of portion B of male rotor 22. Further, it will be appreciatedthat the lead ratio of portion C to portion D, the wrap angles of eachportion and the total wrap angle will correspond with that of the malerotor, or vice versa.

The rotor members may be formed or produced in any suitable manner andpreferably, are constructed in the manner illustrated in FIG. 5. In thisrespect, a male rotor 50 is illustrated which includes shaft means 52carrying helical land and intervening helical groove means 54 defined byhelical land and groove means portions 56 and 58. Portion 56 is integralwith shaft means 52, and portion 58 is defined by a separate componentsuitably interconnected with portion 56. In the particular embodimentillustrated, portion 58 is an annular component having helical land andgroove means in the outer surface thereof and having a cylindricalaperture 60 therethrough which is adapted to receive a cylindricalportion 62 of shaft means 52 which projects from the corresponding endof land and groove portion 56. Portion 58 is interconnected with portion56 by bolt means 64, or the like, which extends through aperture 65 inportion.58 and has a threaded inner end disposed in a cooperativelythreaded recess 66 in portion 56. Further, means is provided to assurethat portion 58 is oriented relative to portion 56 so that the helicalland and groove means of the two portions are disposed in matingrelationship in the transition area therebetween defined by a line ofjuncture G. In this respect, portions 56 and 58 may be provided with arecess 68 and aperture 70, respectively, adapted to be aligned toreceive a suitable dowel pin 72. It will be appreciated, of course, thatrecess 68 and aperture 70 are provided in the corresponding portions 56and 58 so as to assure properalignment of the lands and grooves of thetwo portions uponinsertion of dowel 72 thereinto; After properpositioning is thus assured, aperture 65 and recess 66 can be providedin the corresponding portions 58 and 56, and bolt means 64 insertedthereinto to complete the assembly. I

A rotor constructed as'illustrated in FIG. 5 is very economical toproduce in'that the shaft means and portion 56 of the helical land andgroove means of the rotor can be turned or otherwise produced as anintegral unit, while portion 58 having helical land and groove means ofa different lead which is constant along its entire length can readilybe turned or otherwise produced as a separate element or component.Thereafter, itis only necessary to position portion 58 on the shaftmeans and against portion 56, orient the two portions for the land andgroove means thereof to mate, fix the positions thereof by means of -thedowel pin and interconnect the two portions by bolt means 64.

While the foregoing rotor structure has been described in conjunctionwith a male rotor, it will be appreciated that the female rotor isproduced in the same manner. Further, it will be appreciated that meansother than a dowel pin can be employed to assure proper alignment of thehelical lands and grooves of the two portions of the rotor. In thisrespect, for example, the-projecting portion of the shaft and theopening in the separate component through which the shaft portionprojects could be provided with aligned keyway means adapted to receivea key element which would properly orient the two rotor portions.Further, the projecting portion of the shaft could be provided with aflat and the opening through the separate component could be providedwith a flat side interrupting the cylindrical contour thereof and whichflat side would cooperate with the flat on the shaft to properly alignthe rotor'portions. Still further, although the helical land and groovemeans of portion 56 of the rotor is illustrated as being integral withshaft means 52, portion 56 could also be a separate component from theshaft means suitably interconnected therewith. It is also contemplatedthat portions 56 and 58 could be separate portions each integral with acorresponding shaft portion. In this instance, the two rotor portionswould be suitably interconnected with the helical land and groove meansthereof in mating relationship and the shaft portions thereof wouldtogether define shaft means for the rotor unit.

In use, complementary male and female rotor elements are disposed in acompressor housing in intermeshing relationship with one another andwith high lead portions A and C thereof toward the'compressor inlet andlow lead portions B and D thereof toward the compressor outlet. In amanner well known, fluid such as air enters the compressor inlet and istransferred therefrom to the compressor outlet by being captured inpockets or cells defined by the helical land and groove means of therotating male and female rotors together with the chambers in which therotors are di'sposed. During such transfer, the pockets in which thefluid is trapped gradually decrease in volume whereby the fluid isgradually compressed and is ultimately exhausted through thecompressor-outlet under high pressure and velocity.

The working relationship of male and female rotors made in accordancewith the present invention is scheare numbered 1 through 4. Similarly,female rotor 24 has helical land and groove me'ans portions C, and Dhaving different leads corresponding, respectively, to the leads ofportions A and B of the male rotor. Further, the helical lands andgrooves of portions C and D are disposed in mating relationship. Thegrooves of female rotor 24 are numbered 1 through 6. The space be tweenlines 73 and 74 of male rotor 22 together with. the space between lines75 and 76 of female rotor 24 define the width of the compressor inlet-Ifthe leads of the helical land and groove means of the rotors wereconstant throughout the lengths thereof the compressor outlet would havea width designated by lines .78. However, by-providing the portions ofthe rotors adjacent the outlet of the compressor with helical land andgroove means having a lead which is less than that of the portionsadjacent the inlet end of the compressor, a compressor outlet of greaterwidth or size as designated by lines 80, advantageously is provided for.The small outlet opening necessitated by the use of constant lead rotorsproduces an undesirably. high output velocity from the compressor, andif the outlet is enlarged to decrease the velocity there is a resultantbackflow V which disadvantageously results in imposing additional workon the compressor and thus decreasing its efficiency. The low leadfeature provided for the outlet ends of the male and female rotors inaccordancewith the present invention allows for the size of thedischarge opening to be increased to reduce the output velocity of thecompressor without the resultant backflow and loss of efficiency.

' While considerable emphasis has been placed herein on the structure ofthe rotors being such that the male rotor has four helical lands andintervening grooves and the female rotor has six helical lands andintervening grooves, it will be appreciated that other well known rotorconfigurations and ratios may be employed.

As many possible embodiments of the present invention may be made and asmany possible changes may be made in the embodiment herein set forth, itis to be distinctly understood that the foregoing description of thepreferred embodiment is to be interpreted merely as illustrative'of thepresent invention and not as a limitation.

I claim: I

1. In a rotary fluid compressor of the character comprising a housinghaving laterally communicating cylindrical chambers in which a pair ofintermeshing male and female rotors are disposed for progressivelycompressing fluid between an inlet port at one end of said chambers andan outlet port at the other end of said chambers, said rotors havingcorresponding inlet and outlet ends with respect to said inlet andoutlet ports, said male rotor having arcuately convex helical lands andsaid female rotor having complementaryv arcuately concave helicalgrooves, said lands andg'rooves being cooperable in response to rotationof said male and female rotors to progressively and continuouslycompress fluid between said inlet and outlet ports within a total wrapangle of the lands and grooves less than 360 and wherein the dischargeport opens into said chambers peripherally of and laterally therebetweenat the discharge ends of the rotors and the lead of said lands andgrooves adjacent said outlet port is less than the lead of said landsand grooves adjacent said inlet port, the improvement comprising: saidmale and female rotors each including structurally separate first andsecond portions having helical lands and grooves, said first portionsextending along said rotors from said inlet ends of the rotors to atransition plane intermediate said inlet and outlet ends and transverseto the axes of said rotors, said second portions extending along saidrotors from said transition plane to said outlet ends, the lands andlgrooves'in said first portions having a constant ,lead, and the landsand grooves in said second portions having a constant lead which is lessthan the constant lead of said first portions, and means interconnectingsaid first and second portions of said rotors for the lands and groovesthereof to engage in mating relationship at said transition plane, saidrotors including shaft means, one of said first and second portionsbeing integral with said shaft means and the other of said first andsecond portions being apertured to receive said shaft means, the ends ofsaid first and second portions at said transition plane having pairs ofaxially aligned recesses therein, at least one of the recesses of thepair in said other portion extending completely through said otherportion, and the recess in said one portion aligned therewith beinginternally threaded, said interconnecting means including a pin in theother of the aligned recesses in said one and other portions and athreaded fastener extending through said one recess into threadedengagement with said threaded recess.

2. The improvement according to claim 1, wherein the ratio of the leadsof said first portions to the leads of said second portions isapproximately 3 to l.

3. The improvement according to claim 1, wherein the wrap angle of eachof said first and said second por tions is approximately 135. 7

4. In a rotary fluid compressor of the character comprising a housinghaving laterally communicating cylindrical chambers in which a pair ofintermeshing male and female rotors are disposed for progressivelycompressing fluid between an inlet port at one end of said chambers andan outlet port at the other end of said chambers, said rotors havingcorresponding inlet and outlet ends with respect to said inlet andoutlet ports, said male rotor having arcuately convex helical lands andsaid female rotor having complementary arcuately concave helicalgrooves, said lands and grooves being cooperable in response to rotationof said male and female rotors to progressively and continuouslycompress fluid between said inlet and outlet ports within a total wrapangle of the lands and grooves less than 360, and wherein the dischargeport opens into said chambers peripherally of and laterally therebetweenat the discharge ends of the rotors and the lead of said lands andgrooves adjacent said outlet port is less than the lead of said landsand grooves adjacent said inlet port, the improvement comprising: saidmale and female rotors each including structurally separate first andsecond portions having helical lands and grooves, said first portionsextending along said rotors from said inlet ends of the rotors to atransition plane intermediate said inlet and outlet ends and transverseto the axes of said rotors, said second portions extending along saidrotors from said transition plane to said outlet ends, the lands andgrooves in said first portions having a constant lead, and the lands andgrooves in said second portions having a constant lead which is lessthan the constant lead of said first portions, and means interconnectingsaid first and second portions of said rotors for the lands and groovesthereof to engage in mating relationship at said transition plane, saidrotors including shaft means, said first portions being integral withsaid shaft means and said second portions being apertured to receivesaid shaft means, the ends of said first and second portions at saidtransition plane having pairs of axially aligned recesses therein, atleast one of the recesses of the pair in said second portion extendingcompletely through said second portion, and the recess in said firstportion aligned therewith being internally threaded, saidinterconnecting means including a pin in the other of the alignedrecesses in said first and second portions and a threaded fastenerextending through said one reangle of each said portions isapproximately

1. In a rotary fluid compressor of the character comprising a housinghaving laterally communicating cylindrical chambers in which a pair ofintermeshing male and female rotors are disposed for progressivelycompressing fluid between an inlet port at one end of said chambers andan outlet port at the other end of said chambers, said rotors havingcorresponding inlet and outlet ends with respect to said inlet andoutlet ports, said male rotor having arcuately convex helical lands andsaid female rotor having complementary arcuately concave helicalgrooves, said lands and grooves being cooperable in response to rotationof said male and female rotors to progressively and continuouslycompress fluid between said inlet and outlet ports within a total wrapangle of the lands and grooves less than 360*, and wherein the dischargeport opens into said chambers peripherally of and laterally therebetweenat the discharge ends of the rotors and the lead of said lands andgrooves adjacent said outlet port is less than the lead of said landsand grooves adjacent said inlet port, the improvement comprising: saidmale and female rotors each including structurally separate first andsecond portions having helical lands and grooves, said first portionsextending along said rotors from said inlet ends of the rotors to atransition plane intermediate said inlet and outlet ends and transverseto the axes of said rotors, said second portions extending along saidrotors from said transition plane to said outlet ends, the lands andgrooves in said first portions having a constant lead, and the lands andgrooves in said second portions having a constant lead which is lessthan the constant lead of said first portions, and means interconnectingsaid first and second portions of said rotors for the lands and groovesthereof to engage in mating relationship at said transition plane, saidrotors including shaft means, one of said first and second portionsbeing integral with said shaft means and the other of said first andsecond portions being apertured to receive said shaft means, the ends ofsaid first and second portions at said transition plane having pairs ofaxially aligned recesses therein, at least one of the recesses of thepair in said other portion extending completely through said otherportion, and the recess in said one portion aligned therewith beinginternally threaded, said interconnecting means including a pin in theother of the aligned recesses in said one and other portions and athreaded fastener extending through said one recess into threadedengagement with said threaded recess.
 2. The improvement according toclaim 1, wherein the ratio of the leads of said first portions to theleads of said second portions is approximately 3 to
 1. 3. Theimprovement according to claim 1, wherein the wrap angle of each of saidfirst and said second portions is approximately 135*.
 4. In a rotaryfluid compressor of the character comprising a housing having laterallycommunicating cylindrical chambers in which a pair of intermeshing maleand female rotors are disposed for progressively compressing fluidbetween an inlet poRt at one end of said chambers and an outlet port atthe other end of said chambers, said rotors having corresponding inletand outlet ends with respect to said inlet and outlet ports, said malerotor having arcuately convex helical lands and said female rotor havingcomplementary arcuately concave helical grooves, said lands and groovesbeing cooperable in response to rotation of said male and female rotorsto progressively and continuously compress fluid between said inlet andoutlet ports within a total wrap angle of the lands and grooves lessthan 360*, and wherein the discharge port opens into said chambersperipherally of and laterally therebetween at the discharge ends of therotors and the lead of said lands and grooves adjacent said outlet portis less than the lead of said lands and grooves adjacent said inletport, the improvement comprising: said male and female rotors eachincluding structurally separate first and second portions having helicallands and grooves, said first portions extending along said rotors fromsaid inlet ends of the rotors to a transition plane intermediate saidinlet and outlet ends and transverse to the axes of said rotors, saidsecond portions extending along said rotors from said transition planeto said outlet ends, the lands and grooves in said first portions havinga constant lead, and the lands and grooves in said second portionshaving a constant lead which is less than the constant lead of saidfirst portions, and means interconnecting said first and second portionsof said rotors for the lands and grooves thereof to engage in matingrelationship at said transition plane, said rotors including shaftmeans, said first portions being integral with said shaft means and saidsecond portions being apertured to receive said shaft means, the ends ofsaid first and second portions at said transition plane having pairs ofaxially aligned recesses therein, at least one of the recesses of thepair in said second portion extending completely through said secondportion, and the recess in said first portion aligned therewith beinginternally threaded, said interconnecting means including a pin in theother of the aligned recesses in said first and second portions and athreaded fastener extending through said one recess into threadedengagement with said threaded recess.
 5. The improvement according toclaim 4, wherein the lead of said first portion is approximately threetimes the lead of said second portion and the wrap angle of each saidportions is approximately 135*.